This invention relates to the purification and decontamination of the secondary coolant of nuclear steam supply system.
In a pressurized water reactor equipped with steam generators that have both liquid and vapor phases, any impurities entering the generators in the feedwater have a tendency to concentrate in the liquid phase. Contaminants in the secondary coolant originate from a number of sources including: feedwater makeup water, corrosion of the secondary system, condenser leakage, steam generator tube leakage, and residue from manufacture and assembly and subsequent cleanups of the steam generator. Several problems occur when contaminants are concentrated to an excessive degree in the steam generator. One danger is that the contaminants tend to plate out on the heat transfer surfaces and tend to settle out on horizontal areas. This results in the formation of scale and layers of crud which decrease the efficiency of heat transfer processes thereby decreasing the heat transfer performance of the steam generator. In addition the formation of scale and crud deposits can have an even greater effect on the steam generators. Recent industry experience and laboratory test results indicate that impurity deposits can act to accelerate the corrosion of steam generator heat exchange tubes. Corrosion of the steam generator tubes is undesirable since excessive corrosion results in steam generator tube failure which allows the leakage of the pressurized radioactive primary coolant into the secondary system thereby contaminating the secondary coolant. Therefore, it is desirable to exclude feedwater impurities from the steam generator liquid phase. It is also desirable to have a system which can remove both radioactive and non-radioactive contaminants from the secondary coolant without incurring excessive cost.
Since solid contaminants have such a pronounced effect on the integrity of the steam generator heat exchange tubes, every attempt should be made to exclude all solids from the steam generators. It has been conventional in the past to attempt to prevent impurity collection in the steam generator by periodically blowing down the steam generator liquid to a holding tank for delivery to a waste treatment facility. Another conventional prior art method has been to add solid chemicals to the coolant for the purpose of combining with the dissolved impurities in the steam generator feedwater to produce a particulate precipitant. The particulate was then later removed by filtration and/or ion exchange. Neither of these prior art systems has proved to be successful in preventing steam generator deposits and subsequent steam generator corrosion. Both approaches allow the collection of solid impurities in the steam generators for at least a short period of time. Therefore, plating out and crud deposition occur regardless of the clean up efforts.